Large centrifugation systems typically use a removable rotor for holding sample containers which contain the sample to be separated. The rotor is covered by a rotor lid and then placed into an instrument chamber wherein the rotor is spun during centrifugation.
A typical centrifuge assembly is shown with reference to FIG. 6. The centrifuge assembly 10 consists of a rotor assembly 12 which is connected by a rotor shaft assembly 14 to a drive motor 16. With reference to FIG. 7, the rotor assembly 12 includes a rotor body 110 which has a rotor chamber consisting of a plurality of chambers 112 for receiving configuration sample containers, not shown, which hold the sample being centrifuged and an interior upper chamber 114. The separation between the canister chambers 112 and the interior upper chamber 114 is shown by a dashed line 113. Interior chamber 114 is the volume which remains within the rotor chamber after the insertion of the centrifugation containers. At the upper end of the rotor body 110 is an annular opening defined by an edge 150. The opening in the top of the rotor body 110 is intended to be covered by a lid assembly 15. Rotor body 110 includes an axial bore 120 formed through the spin axis of the rotor body, extending from an open end within interior chamber 114 to an open end 125 at the bottom of the rotor body. Axial bore 120 includes one or more locking pins 130 which project into the interior volume of the axial bore.
Setting up the rotor assembly for a centrifugation run includes placing the rotor into a instrument chamber, not shown. The instrument chamber includes a spindle hub 20 which is part of the rotor shaft assembly 14 and is received in the axial bore 120 of the rotor body 110. The inserting end of the spindle hub 20 is slotted to engage locking pins 130, thus locking the spindle hub into position relative to the rotor body. The spindle hub 20 is coupled to the quill shaft 22 and the bottom of the shaft is coupled to a drive motor, or other type of rotation source, which provides the torque to spin the rotor. A housing 26 encloses a large portion of the shaft and forms the outer enclosure of the rotor shaft assembly 14.
A common problem that exists in centrifugation systems is that the centrifuge rotor becomes unbalanced and vibrates when the rotational speed in the system reaches a critical speed. Normally, the rotor rotates about its geometric center of gravity. As the speed of the rotor increases, it reaches a critical speed which is defined as "the angular speed at which a rotating shaft becomes dynamically unstable with large lateral amplitudes due to resonance with the natural frequencies of the lateral vibration of the shaft." (McGraw-Hill Dictionary of Scientific and Technical Terms, 5th Edition, 1994.) At the critical speed, the rotor experiences a radial displacement that is synchronous with rotation, the maximum displacement being at the critical speed. As the speed continues to increase beyond the critical speed, the radial displacement decreases until the only radial displacement remaining is that associated with rotor imbalance. Additionally, since the centrifuge operator must load the samples into the chambers, this can create a further imbalance to the rotating system, which can magnify the radial displacement and vibrating effect. During normal use, the rotor generally passes through its critical speed when accelerating from a stopped position to its normal operating speed and, after centrifugation is completed, when decelerating back to a stopped position.
When designing a shaft for a rotor assembly used in a centrifuge system, the design objectives are usually in conflict. One objective is to have a very flexible shaft in order to minimize bearing loads at super-critical speeds. A second objective is to have a stiff shaft in order to minimize rotor displacement at the critical speed. In conventional designs, the stiffness of the shaft is usually chosen as a compromise between these two competing objectives. Generally in the prior art, the stiffness of the shaft has been designed to have a linear characteristic in that the shaft would have the same stiffness at any rotor speed.
With reference to FIG. 8, rotor shaft assemblies in the prior art generally have a linear stiffness characteristic. As shown in FIG. 8, as the amount of radial displacement, shown on the x-axis, increases, the amount of force resisting the displacement or stiffness, shown on the y-axis, also increases proportionally, resulting in a graph 71 that is a straight line.
In U.S. Pat. No. 5,683,341 to Giebeler, there is a discussion of a prior art shaft design which has a high amount of stiffness for enabling the rotor to be maintained vertically aligned even though some imbalance forces are present. This design is then compared to the design of Giebeler which relies on the rotor being supported above its center of gravity to keep the rotor upright, rather than relying on the stiffness of the shaft. U.S. Pat. No. 5,827,168 to Howell attempts to minimize the vibrations of a centrifuge by using sliding and damping bearings to restrain vertical movement of a disk rotatably attached to the centrifuge's drive shaft.
It is the object of the present invention to provide a rotor shaft assembly having an improved characteristic of stiffness, such that the shaft can maintain the required stiffness to compensate for rotor displacement at the critical speed, but can be flexible at super-critical speeds in order to minimize bearing loads.